I usually would just call the setting I was looking for and give them a little spiel:
"My name is X, i am a pre-physical therapy student from X university looking to obtain observation hours. I was wondering if it would be possible to do X hours at your site?"
That's how I got my observation hours in. At the hospital, they said yes but then referred me to the hospitals volunteer department. I had to become a certified hospital volunteer (complete with immunizations and training) before I could even shadow. It was a lot of work, but I was able to get a lot of hours in and had a wonderful experience.
At my school, they said to be sure to use the term observation. Saying volunteer may make the therapists feel as if they have to have some type of job for you to be doing when you are there and they may not want to provide that. Always say observation or shadow 🙂
--Another key: be persistent. You may call a place and get a voicemail box. Leave one. Then follow up if you don't hear back in a few days. You may feel you are annoying but I've had a therapist tell me specifically it was my persistence that made them interested in having me around--they could tell I was serious and interested in the profession!
Hope that helps and good luck. Remember, the PTs were students once too. They understand we need to have experience in the field and most of them are more than happy to help a student out.